This invention relates generally to well logging utilizing nuclear magnetic resonance, and more particularly to apparatus and methods for utilizing nuclear magnetic resonance to log a well as it is drilled.
The derivation of information relating to petroleum or other organic fuel deposits at an underground geological location is the subject of various technological approaches. For example, one common prior art technique to derive such information is known as nuclear magnetic resonance (NMR) logging. Basically, the NMR logging technique entails introducing a nuclear magnetic resonance logging apparatus or probe into a well or bore hole which has been drilled by conventional drilling equipment. The probe serves to excite the nuclei of materials in the surrounding geological structure so that various parameters, e.g., spin density, T.sub.1 and T.sub.2 relaxation, et., of the surrounding geological formation can be measured. From that data valuable information regarding the makeup of the structure, e.g., the amount of extractable oil, can be determined.
Examples of prior art NMR logging apparatus and methods are described in the following U.S. Pat. Nos. 3,508,438 (Alger et al), 3,617,867 (Herzog), 3,667,035 (Slichter), 4,350,955 (Jackson), 4,467,642 (Givens), 4,528,508 (Vail, III), 4,560,663 (Nicksic et al), 4,629,986 (Clow et al), 4,710,713 (Taicher et al), 4,713,881 (Givens), 4,717,878 (Taicher et al), and 4,933,638 (Kenyon et al).
While the apparatus and techniques disclosed above appear generally suitable for their intended purposes, they nevertheless suffer from one significant drawback. That drawback is that such prior art systems are not designed (and are generally unsuitable) for effecting the logging of the bore hole as it is drilled. Instead they are designed such that the bore hole is drilled first, the downhole drilling apparatus is then removed, and then the logging apparatus or probe is introduced into the bore hole and their measurements then are made therewith.
A much more desirable technology would permit the logging of the desired rock parameters during the actual drilling process. The measuring of such parameters of the geological structure adjacent the bore hole as the bore hole is drilled is sometimes referred to in the art as 37 measuring while drilling" (or its acronym "MWD") or "logging while drilling" (or its acronym "LWD"). Such procedure has been accomplished heretofore utilizing non-NMR based apparatus. For example, Teleco Oil Field Services, Inc., of 105 Pondview Drive, Meriden Conn. 06450, offers for sale a tool under the name "RGD (resistivity-gamma-directional) MWD tool". That device is described by Teleco as a multi-sensor tool which transmits real-time measurements to surface equipment for detection, decoding, displaying and archiving. The RGD tool consists of three downhole sensors installed in a forty-foot drill collar. The formation resistivity sensor and the gamma ray sensor are both located in a subassembly below the directional sensor and transmitter, and are placed as close as possible to the drill bit. The integrated assembly offers two options, namely, downhole data recording, or transmission of the MWD data uphole in the form of coded pressure pulses in the drilling mud. Data from the downhole tool and surface information (such as depth, rate-of-penetration, hookload and rpm) is then recorded by a range of cabin and non-cabin based surface data acquisition systems. Such surface systems present data on real-time logs and archive the data on either tape, floppy disk or both.
The major components of the downhole RGD tool include a transmitter which produces positive mud pulses in the drilling mud in response to digitally coded signals from the sensor. A mud circulation driven turbine provides power for the downhole assembly. The turbines are configured to manage flow ranges from 250 GPM to 1,100 GPM. A resistivity sensor is provided which is a sixteen-inch spacing short-normal resistivity sensor located in a subassembly below the transmitter and directional sensor. The natural gamma ray sensor Teleco provides is located midway between the resistivity electrodes so that both formation evaluation sensors simultaneously obtain their information at the same depth, thus eliminating the need to memorize either measurement. A tool orientation system is also provided which is comprised of a digital sensor system which measures tool orientation information (e.g., azimuth, and inclination).
Such downhole assemblies (DHA's) are configured inside a drill collar. MWD collars allow the drilling mud to flow around the various sensors. Most sensors are located in the wall of the drill collar adjacent the outer surface and often need electrical isolation from the collar.
While the foregoing tools may offer the advantage over wireline logging technology by enabling logging to be accomplished while drilling, such tools nevertheless suffer from certain natural limitations. Such limitations result primarily because they attempt to replicate the wireline logging tools which operate in a much more gentle environment than do MWD tools. The MWD/LWD tools require more complex, robust technology and thus they tend to be more expensive. To do an adequate petrophysical evaluation, a minimum of three types of measurements or logs are required, i.e., neutron, deep resistivity, and density. Moreover, such systems may not be sufficient to predict the amount of petroleum present because they lack certain key information, and thus are unable to predict the productivity qualities of the rocks.
NMR logging apparatus and techniques address those limitations by providing direct measurements of this key information. When synergized with the resistivity, gamma and directional MWD tools, a full petrophysical evaluation is possible. The value of this NMR apparatus is clearly demonstrated by comparing it to the wireline technology's ability to provide information on the pore structure and the fluids present. This is possible because of the NMR's ability to assess the rock's porosity system independent of the rock formation type. However, as mentioned earlier, prior art NMR logging systems have not permitted adoption of their measuring system to MWD/LWD technology.